Maddog's Lamington Jr Amplifier build diary

[Maddogm]

and as I was packing up I found the addendum sheet which said to fix instability on some build they'd replaced 10k's with 100k's. but the resistors being referred to (R15 & R17) are actually the 4k7's.

That would be why I didn't pick it up when going through the parts list

[Maddogm]

So just to add to the confusion, R15 & R17 are listed on the schematic as 10k. But in the instructions only the values are used, not the R numbers. So R15 is referred to as a 4k7 and R17 is referred to as a 10k. Just to make it more fun R3 & R11 are actually 10k

So hopefully you're all now as confused as I am.

[Simon Barden]

So 4k7 = 10k = 100k. Easy.

Obviously their values aren't mission critical and it will still make a noise regardless.

I'd add in some 3-way switches and use all three values in those positions, and switch between them to see what sounds best.

[DrNomis_44]

I'm guessing that the 4k7, 10k, and 100k resistors are probably the negative feedback resistors, in that case, the larger the value the more gain that the power amp will have, but the total harmonic distortion of the power amp will increase too.

[Marcel]

On the circuit I downloaded the resistors marked as R15 & R17 are grid stopper resistors to the push/pull output stage 6BL8's. There should not be a lot of audible difference between 4k7 or 10k however as they have recommended 10k resistors for stability then that is what you should preferably use.

FYI - In most EL84 push/pull circuits of similar design to the Lammington these grid stopper resistors can be anywhere from 2k7 to 15k however 10k is frequently used. Larger tubes like EL34's often have 4k7 or 5k6 grid stopper resistors. The exact final installed value is highly dependant on the applied DC plate voltage and current, idle cathode voltage and the tube load which is most often determined by the output transformer design and the connected speakers. Their main function is to limit grid current if ever there is excessive positive voltage applied to the output stage grid from the previous phase splitter stage. Instability often manifests itself as self oscillation of one or both of the output tubes, or alternately dramatically shortened life from either or both output tubes.

[DrNomis_44]

My Marshall MA100C uses four EL34 power valves and each one has a 5k6 grid-stopper resistor on it.

Here's a tip, if you want to stabilize the phase-splitter stage, you can solder a small value cap, say a 100pF/3kV blue disc ceramic cap, across from one anode of the phase-splitter 12AX7 valve to the other anode (pins 1 and 6 of the 9-pin socket), I've seen a Mesa/Boogie Mk I circuit diagram where such a cap was included.

[Marcel]

Most Marshall designs that I've seen have a 47pF 600V cap between the two anodes of the phase splitter. Did the same in my home made EL34 based 50W amp.

Interestingly, in a AC30 the VOX people use a 4n7 and a 250k pot in series across the other side of the two caps that go to the EL84 o/p stage grids (with 1k5 grid stopper resistors), and call that knob a 'Treble cut'...

[DrNomis_44]

Most Marshall designs that I've seen have a 47pF 600V cap between the two anodes of the phase splitter. Did the same in my home made EL34 based 50W amp.

Interestingly, in a AC30 the VOX people use a 4n7 and a 250k pot in series across the other side of the two caps that go to the EL84 o/p stage grids (with 1k5 grid stopper resistors), and call that knob a 'Treble cut'...

I remember looking at a circuit diagram of a Vox AC30 Top Boost, and seeing the Top Cut tone circuit and then thinking it was a bit unusual to see something like that in a guitar amp circuit, since I was used to seeing the usual three band EQ tone-stack circuit as found in Fender and Marshall amps.

[Marcel]

The more I look at tube amp circuits the more they all seem the same to me.

Every one of them use the same or very similar techniques to get a result, That being said, a look at any particular brand will reveal consistencies across most of their models thus contributing to their sound. Pre-amp tubes and power tubes biased a certain way and/or of specific types, tone stacks configured in a specific manner with usual values, bypass caps being typical types and values, plus many other unique features all contribute to that manufacturers typical sound.

So I'm often left with a question in my head that leaves me searching... What is it about a particular model that makes certain genres of guitarist proclaim loud and wide that this or that model is the best for what they play?

All too many times I read on other forums that a guitarist loves model XYZ of a brand but loathes the model ZYX of the same brand yet the circuits are identical. It makes me laugh...and I conclude that it must be the speaker cab, or the age of the caps and tubes, or a minor fault in the amps he is using that has set that guitarists heart ablaze...

I spent much of last night reading the http://sound.whsites.net/index2.html site. Most of the time I was where he talks about tubes and tube amps. He is a HiFi guy that has a lot of good knowledge to offer, but has yet to realise that when it comes to guitar amps the guitarist is not after perfect fidelity or a low THD or IMD. Be they tube or SS it doesn't matter, it is the sum of all those many parts with or without all their faults to sound awesome that the guitarist seeks. A few notes from a mildly distorted Strat into a Phaser and a Echorec and then a small WEM 15W tube amp are instantly recognisable by most as the intro to 'Shine on you crazy diamond', yet the Bass lines to most modern music today is nearly all pure SS... so to a guitarist it is the total combination that counts, not the tubes alone.

[Chuck]

Hey Maddogm, I'm following this thread closely as about 8 months ago I bought the same kit. Just haven't got around to building it yet!! One thing I want to do is not use the lamington tin, but make an aluminium enclosure for it. There are a couple of bits that look like they've been expoyed to the tin - do you think it would be possible to get them off and then re-epoxy them to a new enclosure? That said, I'm not even sure how you'd get them off! Here's a pic of the offending parts on my kit...